June 3, 2016

Team Duck's Forecast:

"A 50 knot midlevel speed maximum associated with a northern stream trough will provide a shortwave impulse over eastern SD and NE NE, Northern Iowa, and Minnesota by the later part of today. In response, the low over western South Dakota will move ENE, toward north central MN. A warm front will extend from central MN into northern WI, while a trailing cold front extends SW over IA and NE. The low’s position further east puts the warm frontal scenario firmly into the tree-filled jungle type area that does not present a welcoming target given our project goals. While the warm front sits along the MN/WI border, this is only during the early day hours (19Z), before it departs for northern/eastern WI - it would seem a less credible target given the reasons stated by Skip. Particularly revealing are the updraft helicity swaths from the 4km NAM found over north central WI. Dew points in NAM are also ridiculously optimistic, with values progged in the 60s - but with no moisture source within the surrounding states in excess of 55-58F (IA in NAM is suggesting 60s now, which don’t exist) - this suggests that instability will be more marginal than first indicated, and is consistent with the SPC analysis which suggests low dew points, meaning this system is going to struggle. More credible is the moisture return along the cold frontal sector, or perhaps in the post frontal airmass, with dews near 60ºF in northern KS being advected toward the cold front. The upper air temperatures (500mb) are ~-18C, though the 850s remain somewhat warm thanks to the persistent ridge, and quite high H10-H85 lapse rates suggest reasonable potential, along with the only reasonable plume of midlevel lapse rates. On the frontal sector, reasonable CAPE with deep layer shear will favor more useful storms in better terrain, and despite mediocre hodographs, there is certainly a non-zero tornado chance.

Both CAMS show supercell-like storms in the post rainband recovery cold frontal airmass in west-southwest MN and have done so persistently, forming around 20Z. In contrast, little to nothing impressive is shown in the rapidly departing warm frontal sector, with little to no active storms expected. In light of this, I favor either a cold front/or post rainband/frontal airmass type target, as the terrain and tree issues along with the poor expectations of seeing any sort of tornado or even a reasonable storm seem remote."

Team Woodchuck's Forecast:

"Slim, but by no means hopeless pickings today. In a nutshell, the only credible play I see is to position in E/SE MN just south of the warm front and look for convection developing along it to acquire supercell structure as it tracks into Wisconsin. The NAM is the Uber-optimist among the model flock this morning, showing a mesolow at 21 UTC along the warm front on the MN-WI border northwest of LaCrosse WI, which yields the sounding shown below. This is a credible scenario, but as far as the likelihood of verification goes, who knows?! It is 4 hrs 20 mins to reach Rochester MN on the interstate; the Mississippi River will have to be considered for chase strategy if we do head out that way."

Team Turtle's Forecast:

"I see two targets, the warm front extending off the developing/weak low in east central MN into WI, and the trailing cold front in central and southwest MN.

I agree with Anton that the best tornado play will be on that warm front. I see number of issues at this target though, the biggest is the terrain. Areas north of 90/94 I generally avoid even if there are significant tornado probabilities, simply because the trees across the region and then the terrain near the river make visibility nearly impossible. Ongoing crapvection on the warm front looks to hamper instability, which most of the models are agreeing with. Looks like it's going to be a slightly earlier show too given how the shortwave ejects. I think we'd have to leave right now, for a mid afternoon intercepts of grungy storms in the trees. I'd only chase this target if we had to give 100% effort for every tornado chance we have as futile as it may be, but expect our chances of seeing a tornado given the area difficulties to be near zero.

Veered winds, mediocre hodos, and possibly storm coverage look to hamper storm mode down the cold front, yet this looks like the more "chaseable" option of the two to me. Decent moisture advection/convergence along this boundary aided by cold air aloft as the trough pivots could make for some very robust updrafts. Updraft velocity and vorticity stretching from a cold air aloft type of situation may compensate somewhat for that bad shear profiles across the target and may get a shot at a tornado. I would target WIndom to Fairmont area along the I-90 corridor for a 23-0z intercept of mixed mode storms, hopefully squeezing a discrete supercell somewhere out of the bunch. Chances are quite low for a tornado intercpet, but this is the more viable intercept option in my opinion. For what it's worth the HRRR is also showing some updraft helicity tracks in this area, even more so than the warm front target."

Team Duck's Summary:

"It was overcast with rain further east as we met for coffee at a Bagel shop that morning. The target was for western MN, so we were in a good position to head east in not to short a time period. A surface low over northern MN draped a cold frontal boundary SSWestwards over the Sioux City region. Weak convection and precipitation were formed along the front, and moved southeastwards throughout the morning, laying down a cool moist outflow boundary to its west along the corridor from Sioux Falls northeastward.

The arrival of cool air in association with the upper low overlying a warm recovery airmass with dew points in the low 60s and SBCAPE between 1500-2000j/kg promoted the growth of robust updrafts. The initial rounds of these were seen at Aberdeen SD on satellite, while strong vertical wind shear formed in association with the arriving trough. The anticipation was for either a cold core localised vorticity landspout type storm, or a tornado if a storm happened to catch and interact with the cooled outflow boundary prior to congealing into a line.

Initial positioning was near Sioux Falls, and we observed the convection developing to our northwest while also watching convection southeast of Chamberlain SD. As a dominant cell began to form ahead of the line to our northwest, the Chamberlain cell developed a more linear complex structure, and hence we drove north toward Brookings. Unfortunately, the strong forcing from the frontal cold core prompted cells to develop on the southwest side of the primary cell, and thus despite retaining rotating updrafts, the cold front and storm outflow boundaries undercut the storm system with cold air, inhibiting tornado potential along most of the structure.

We would later learn that a tornado had formed on the outflow boundary interface to the northeast of our target cell, with turkeys as debris. Returning south to Sioux Falls after evaluating the situation, we decided to call the chase, as no viable storms remained within the target region. We drove west back through some heavy rain and outflow gusts along with witnessing some impressive lightning strikes. Dinner was at a Mexican restaurant, and was a very enjoyable affair."